Magnetic compass



April 21, 1942.

' E. A. SPERRY'. JR

MAGNETIC comlxss Filed June 10, 1939 INVENTOR ELMER A. sPEfiRXM ATTORNEY Patented Apr. 21, 1942 Elmer A. Sperry, In, New York, N. Y., assignor to Sperry Products, Inc., Hoboken, N.'J., a corporation of New York Application June 10, 1939, Serial No. 278,443

6 Claims. (0!. s s-e23) This invention relates to magnetic compasses and has for its principal object the provision of a compass which will enable the pilot to navigate a course, particularly a straight course, without having the compass subjected to the recurring errors due to acceleration forces. The difilcul'ty attending the flying of a straight course by magnetic compasses has been fully' set "forth'ln the application of Charles Stark Draper and Walter McKay, Serial No. 186,156, filed January 21, 1938-, and, briefly stated, this difllculty is known as the northerly turning error and is defined. as'that error which is experienced by the compass card during flight of the craft upon any course having a northerly or southerly component. The error manifests itself during turns, or even during yawing and rolling movements attendant on straight flight in any but perfectly smooth air, by turning of the compass card in the same direction as the craft when traveling on a course having a northerly component and, in the opposite direction when trav'elingon a course having a southerly component, and at a faster rate in the initial instants, hence indicating to the pilots a turn in the opposite direction to that his craft is making. Therefore the pilot watches his other instruments, such as histurn indicator and directional gyro, to fly a straight course, since these instruments detect turns away from said course. It is obvious, however, that if the northerly turnme error could be substantially eliminated from the magnetic compass the pilot would be able to detect deviations from a straight course.

In the said patent application of Draper and McKay there was disclosed the general solution ,ior the elimination of the turning error, said solution consisting in providing a magnetic com-' pass having a meridian seeking'elernent wherein the undamped Z period (that is, the period around the vert cal axis) is in excess of a critical point. It is the principal object of this invention to provide a specific construction of meridian seeking element which will fulfil substantially the hereinbefore specified conditions for the practical elimination of the northerly turning error.

Further objects and advantages of this invention will become apparent in the iollowing detailed description.

In the accompanying drawing,

container in having a cover it and a transparent side wall [3" forming a fluid-tight compartment adapted to be filled with fluid. Within the cas'-' ing there is supported a sensitive element 5- comprising a float chamber 14, apivot l6, magnets H, a-ce'n'tralplate'iB, an outer rim 19 formed radial with respect to said pivot, and a. plurality of vanes 20 supported between said central plate and said outer rim. The vanes are arranged vertically so that they enclose the body of liquid between them laterally but present no obstruction to the liquid vertically." Thatis to say, the

sensitive element in the portion which, includes the vertical vanes '20 is completely open top and bottom to offer practically no resistance to any movements of the sensitive element around the X and Y axes; that is, around any horizontal axes. The radial formation of the rim also facil- Fig. l is a vertical section through a magnetic compass embodying my invention.

Fig. 2 is a perspective view of the sensitive element.

Referring first to Fig. l'there is disclosed a itates movement around the X and Y axes because as so formed the liquid offers practically no resistance to the movement of the rim. Movem'ent around the vertical or Z axis, however, will cause the vertical vanes 20 to tend to carry the full body of liquid enclosed between adjacent members 20, thus increasing greatly the moment of inertia around the Z axis. Therefore the tinclamped Z period of the compass is substantially increased while the moments of inertia of the sensitive element around the X and Y axes, and therefore the undamped periods about these axes, are substantially the same as if the vertical vaneswere not employed..- The Z p'erlod will be hot" less than 3 minutes while the X and Y periods will by 2 or 3'seconds.

The above sensitive element,v therefore, offers a desirable embodiment of the theory set forth in the said Draper. and McKay patent applica-=-' tion in that the northerly turning error is substantia'lly eliminated because of a eatly in creased undamped Z periodwhile retainln'grelw' tively fast X and Y periods. The latter are desirable because" if the X and Y periods were. also increased the card would oscillate excessively in tilt. I have thus provided a compass wherein the undamped Z period. is made relatively long while the und'a-m'ped X and Y periods remain relatively short, thus not only substantially alibiinating the northerly turning error but also resuiting in aminimumosciliat'ion of the card-relative to the craft because the cardbanks Withthe craft due to the relatively fast X and'Y period's. f

Since the sensitive element has a slow'period around the Z axis it is desirable that the liquid have low viscosity on the order of less than 1 centistoke to prevent swirl of the'liqu'id from afiecting the card. Further, it is desirable that the liquid have a high density, greater than ll. This is for the purpose of increasing the moment of inertia of the sensitive element around the Z axis. Further, the high density permits a small displacement of the sensitive element and therefore there will be but small variation of pivot load when there is change in temperature .of the density 1.35. Such liquids will enable a sensitive el ment of small displacement to be used.

The rim i9 and'vanes are preferably made of a light-material such as magnesium or aluminii'm whereby asmal'l moment of inertia around fast Xand"Yperiods.

Thesensitiveelement may be mounted in the usdarman er upon'abearing 2| supported in any nner at 23 upon a support 22 within OT' The rim-1910f the sensitive eleprovided with azimuth indications which 'may be read'in conjunction with a lubber line 25fseen through the'transparent window i3.-

Ah, expansioiich'ainber an may be provided in the usu'al manne'r ommunicating with the chamber infwhich'ithesensitive' element operates, said chamber having an exp'ansible bellows-3 i therein for "taking up the expansion and contraction of the liquid within the casing.

In,accordance with the provisions of the patent statutes, I have herein described the principle and operation or my invention, together with the apparatus which I now consider to represent the best embodiment thereof, but I'deslre to have it understood that the apparatus shown is-only illustrative and that the invention can be carried out by other equivalent means. Also, while it is designed to use the various features and elements in the combination andrelations described, some of these may be altered and others omitted with- -c utint erfering with the more general results outlined, and the invention extends to such use.

Having described my invention, what I claim and desire to secure by Letters Patent is:

.11. A magnetic compass adapted to be mounted upon a craft, said compass comprising a casing filled w'ith jliquid, a magnetic meridian seeking element, and means for supporting said element in said liquid, said element having means for increasing the period about 'the vertical axis withoutv substantially increasing the moments of inertia'about the'horizofital axes, said means comprisingmeanstending to move the liquid enclosed within the "element bodily with said element during movements of the element around the vertical axis biit adapted to move relative to said liquid enclosed within the element during movethe Xand Y .ax esisobtained'and this permits ments of the element around the horizontal axes.

2. A magnetic compass adapted to be mounted upon a craft, said compass comprising a casing filled with liquid, a magnetic meridian seeking element, and means for supporting said element in said liquid, said element having a plurality of vertical vanes substantially radial of the vertical axis of said element, the spaces between said vanes being open at the-top and bottom whereby said vanes tend to maintain the liquid enclosed between the vanes bodily as a unit with said element during relative movements of the element and easing around the vertical axis but move relative to said liquid enclosed between the vanes during movements'oi' the element around the horizontal axes.

3. A magnetic compass adapted to be mounted on a craft, said compass comprising a casing filled with liquid, a magnetic meridian seeking element, and means for pivotally supporting said element for universal movement in said liquid, said element comprising a central portion, an cuter'rim spaced from said central portion, and a plurality of vanes in vertical planes substantially radial oi the vertical axis of said element and forming the sole connection betwen said rim and said central portion.

4. A magnetic compass adapted to be mounted on a craft, said compass comprising a. casing filled with liquid, a magnetic meridian seeking element, and means for pivotally supporting said element for universal movement in said liquid, said element comprising a central portion, an

- outer rim spaced from said central portion, said rim being-formed as part of a sphere having said pivot as its center, and a plurality of vanes in vertical planes substantially radial of the vertical axis of said element and forming the sole connection betwen said rim and said central portion.

5. A magnetic compass adapted to be mounted on a craft comprising a casing filled with liquid,

a magnetic meridian seeking element universally mounted in said casing, said element having.v

means cooperating with the liquid to move a portion thereof bodily with the element in'its movements about the vertical axis to impart to said element an undamped period about said vertical axis adequate for substantially eliminating northerly turning error, said means being substantially inefiective to move any of said liquid therewith during movement of the element about said horizontal axes.

6. A magnetic compass adapted to be mounted on a craft, said compass comprising a. casing filled with liquid, a magnetic meridian seeking element and means for universally supporting said element in said casing, said element having means for moving a substantial portion of the liquid bodily therewith during movements of the element around the vertical axis and said element being adapted to move substantially none of said liquid during movementsof the element around the horizontal axes, whereby a relatively long undamped period around the vertical axis is obtained without substantial increase of the moments of inertia around the horizontal axes.

ELMER A. SPERRY, Jn. 

